Ever stricter environmental standards for sulfur dioxide and particle emission controls during the combustion of coal have created a need for efficient cleaning of coal. In addition, coal with extremely low levels of sulfur and ash could be used as a direct replacement for imported petroleum in many applications. Over 60% of the coal mined in the eastern United States is presently cleaned at the mine site; however, many cleaning processes typically reduce only the ash content of the coal in order to decrease transportation costs and provide a consistent product. The commonly used washing procedures decrease pyritic sulfur levels only moderately, but organic sulfur is unaffected. However, deeper physical cleaning of coal with dense media, for example, can reduce pyritic sulfur levels by a significant amount; but organic sulfur, which generally comprises about half of the sulfur in coal, is not touched even by the most efficient physical cleaning. Thus, to obtain very low levels of sulfur and ash-forming minerals in coal requires chemical cleaning.
Molten caustic leaching (MCL) is an advanced chemical cleaning process for the removal of over 90% of the sulfur and ash from coal, see Meyers, U.S. Pat. No. 4,545,891 issued Oct. 8, 1985. However, during molten caustic leaching a significant fraction of the caustic is converted to carbonate by reaction with carbon from the coal and with oxygen from various sources. Recent tests have shown that a significant source of the oxygen is from the coal itself.
Carbonate formation is detrimental because: (1) caustic is an expensive reagent and carbonate formation leads to its consumption; (2) carbon is the main energy-containing constituent of coal and carbonate formation leads to its consumption; and, (3) carbonate present in spent caustic causes significant difficulties during reprocessing, e.g., it precipitates during countercurrent washing. In short, conversion of caustic to carbonate increases the expense of coal cleaning. For example, when one ton of coal is cleaned with two tons of caustic, each 1% of caustic converted to carbonate adds somewhere between $2.00 and $4.00 to the cleaning costs.
Nevertheless, in spite of the increased cost and the problems attendant with carbonate formation, molten caustic leaching has been demonstrated to be able to reduce ash levels in coal by more than 95% and sulfur levels by as much as 90%. This is the only chemical cleaning process at an advanced stage of development that can also reduce organic sulfur levels by a significant amount. In the MCL process the feed coal is usually physically pre-cleaned before MCL to reduce its mineral content and thereby reduce caustic consumption during MCL.
The coal is mixed with caustic and heated for 1-3 hours at 350.degree. C. to 400.degree. C. Various ratios of caustic to coal have been tried, but about 2 parts caustic to 1 part coal are typically used in unstirred leaching reactors. The caustic can be either NaOH alone or 1:1 mixtures (by wt.) of NaOH and KOH. During the MCL treatment, ash-forming minerals are rapidly converted to soluble salts, while organic sulfur compounds react somewhat more slowly. After leaching, the coal-caustic mixture is washed with water to remove the soluble salts and then with dilute acid to remove ionically bound alkali metals and base-insoluble species such as iron hydroxides. The coal is then finally washed with water to remove any residual acid.
A countercurrent washing procedure may be used for the water washing of the coal-caustic mixture and for the acid washing step or concurrent washings may be used. Countercurrent washing is more efficient.
During the MCL process, various by-products are formed. For example, carbonate is formed by reaction of coal and caustic with some source of oxygen. Volatile organic liquids and gases are released from the coal, while water-soluble organic materials referred to as humic acids are dissolved in the caustic. The liquids and gases are recoverable as a potential source of fuel or for other higher value uses. The humic acids are not readily recoverable form spent caustic solutions, but at the present stages of development of the MCL process, the levels appear to be too low to have significant effects on the overall process.
However, as earlier explained, the production of carbonate during MCL processing is a significant detrimental factor. Formation of carbonate obviously results in the loss of carbon from the coal and in the excessive consumption of caustic, which must be subsequently regenerated by liming. In addition, carbonate is not very soluble in 50% caustic solutions produced from countercurrent washing of coal. Thus, carbonate in excess of solubility in the spent caustic is carried with the coal to a point where it can build up until it finally clogs the filters and brings, for example, the countercurrent washing to a halt.
It is a primary objective of the present invention to develop a process suitable for pretreating the coal prior to molten caustic leaching to decrease carbonate formation during the leaching process.
Another objective of the present invention is to develop an improved, molten caustic leaching process which cleans coal without the attendant difficulties of formation of significant amounts of carbonate.
Another objective of the present invention is to reduce cleaning costs of coal in processes of molten caustic leaching.
The method and manner of accomplishing each of the above objectives will become apparent from the detailed description of the invention which followed hereinafter.